The present invention relates to a fixing device for heating and pressing a sheet with a toner image, and fixing the toner image onto the sheet, especially to a fixing device using a fixing belt and an image forming apparatus provided with the aforementioned fixing device.
In the conventional art, as the fixing device used in the image forming apparatus using electrophotographic process in a photocopier, printer, fax machine, and multifunction machine provided with the functions thereof, the fixing device of heating roller system (also referred to as a heat-fixing roller system) has been employed over an extensive range from low-speed to high-speed machines as well as from monochromatic to color machines. In the fixing device of heating roller system, heat and pressure are applied to the transfer material with an unfixed toner image formed thereon, by a fixing nip portion made up of a heating roller kept at a predetermined temperature, and a pressure roller having an elastic layer and being pressed against the heating roller, while the transfer material is sandwiched and conveyed thereby.
Widening the fixing nip portion is required to heat toner efficiently on the sheet surface by the demand for colorization and speedup in late years. In this case, viewing the constitution of the aforementioned fixing device of a heat roller system, a means for enlarging the diameters of the two rollers or increasing the pressurizing force between the rollers, thereby increasing the crush (distortion) amount of the rollers would be considered to widen the nip portion. However, when such a method is employed, there may arise a problem with respect to the enlargement of the size of the fixing device and the reduction in durability of the fixing device and the degree of freedom of design conditions is low.
As a method for solving this problem, a fixing device of a belt nip system having an rotating endless fixing belt driven by a roller and a pressing pad fixed on the inner circumferential surface side of the fixing belt and pressing the fixing belt toward the heating roller by the pressing pad has been adopted in recent years. In the fixing device of a belt nip system, from the viewpoint of its constitution, the width of fixing nip portion can be set to be wider than in the heating roller system without growing in size.
The fixing device of nip belt system is designed in such a way that the fixing belt is pressed against the heating roller by means of a pressing member. Accordingly, if there is a great friction between the inner surface of the fixing belt and the pressure member, traveling of the fixing belt is blocked, and this will cause image misregistration or sheet wrinkles.
In an attempt to solve such problems, Unexamined Japanese Patent Application Publication No. 2002-148970 discloses a fixing device to prevent image misregistration from occurring, wherein the aforementioned fixing device is provided with a sheet-shaped member to cover the pressure member for the purpose of reducing the drag of friction with the inner circumferential surface of the fixing belt, and large irregularities are formed on the surface of the sheet-shaped member.
In the fixing device disclosed in the Unexamined Japanese Patent Application Publication No. 2002-148970, however, as a result of repeated use thereof, a lubricant coated on the inner circumferential surface of the fixing belt or a releasing agent coated on the surface of the heating roller spreads inside the sliding sheet from the end part thereof, and sticks to the pressure pad covered by the sliding sheet. If the oil of the releasing agent or others sticks to the pressure pad, the pressure pad is subjected to swelling deformation, and the anticipated functions cannot be performed. This problem has been left unsolved in the conventional art.
To solve the aforementioned problem, the fixing device disclosed in the Unexamined Japanese Patent Application Publication No. 2007-79183 provides an outer extension of the sliding sheet. The end parts thereof are located outside the end parts of the fixing roller and fixing belt to ensure that the releasing agent does not easily spread inside the sliding sheet.
However, the addition of the aforementioned extension causes the sliding sheet to be longer than the fixing belt. Friction between the sliding sheet and the end part of the fixing belt accelerates the abrasion of the sliding sheet, with the result that the sheet tends to break more easily.
An object of the present invention is to solve the aforementioned problems and to provide a fixing device using a fixing belt that ensures that the sliding sheet covering the pressure member which presses the inner circumferential surface of the fixing belt is not broken by friction with the end parts of the fixing belt.